The principle of PWM regulation is well known in the art. PWM regulators operate on the basic principle that the incoming DC voltage, which is to be regulated, is chopped or pulsed by means of switched devices (e.g. transistors). The rectangular pulses thus obtained are applied, via a transformer, to a rectifier circuit (including a smoothing filter) for producing the desired regulated DC output voltage. By converting the input voltage into a train of pulses, the magnitude of the rectified output voltage can be regulated by varying the width of the pulses while maintaining the frequency of the pulses constant. Automatic control of such a system is accomplished by monitoring the magnitude of the DC output voltage and by providing suitable circuitry to vary the width of the pulses, as required, in order to maintain the magnitude of the DC output voltage at the required level.
Examples of prior art devices include those depicted in the following U.S. patents, and attention is directed to them:
U.S. Pat. Nos. 3,670,234 dated June 13, 1972 to James M. Joyce; PA0 3,789,288 dated Jan. 29, 1974 to B. H. Assow, et al; PA0 3,806,791 dated Apr. 23, 1974 to Leo J. Johnson; PA0 3,870,943 dated Mar. 11, 1975 to H. R. Weischedel, et al; and PA0 3,988,661 dated Oct. 26, 1976 to Daniel J. McCoy.
One problem existing in prior art circuits employing PWM regulators is that of maintaining current balance between the alternately switched devices (e.g., transistors) which convert the DC input voltage into a train of pulses. These switched devices form the inverter portion of the power supply, and for optimum performance from the circuit, the power handled by each such switched device should be equal to the power handled by any other switched device in the circuit.
Another problem in prior art circuits occurs when multiple power supplies are connected in parallel to supply higher power to a common load. Prior art circuits, for power sharing between several power supplies, have been complex and cumbersome.